Franco Gaetani

The Protective Role of L-Carnitine against Neurotoxicity Evoked by Drug of Abuse, Methamphetamine, Could Be Related to Mitochondrial Dysfunction

Abstract: There is growing evidence that suggests that brain injury after amphetamine and methamphetamine (METH) administration is due to an increase in free radical formation and mitochondrial damage, which leads to a failure of cellular energy metabolism followed by a secondary excitotoxicity. Neuronal degeneration caused by drugs of abuse is also associated with decreased ATP synthesis. Defective mitochondrial oxidative phosphorylation and metabolic compromise also play an important role in atherogenesis, in the pathogenesis of Alzheimers disease, Parkinsons disease, diabetes, and aging. The energy deficits in the central nervous system can lead to the generation of reactive oxygen and nitrogen species as indicated by increased activity of the free radical scavenging enzymes like catalase and superoxide dismutase. The METH‐induced dopaminergic neurotoxicity may be mediated by the generation of peroxynitrite and can be protected by antioxidants selenium, melatonin, and selective nNOS inhibitor, 7‐nitroindazole. L‐Carnitine (LC) is well known to carry long‐chain fatty acyl groups into mitochondria for β‐oxidation. It also plays a protective role in 3‐nitropropioinc acid (3‐NPA)‐induced neurotoxicity as demonstrated in vitro and in vivo. LC has also been utilized in detoxification efforts in fatty acid‐related metabolic disorders.

Possible Mechanism for the Neuroprotective Effects of l‐Carnitine on Methamphetamine‐Evoked Neurotoxicity

Abstract: Some of the damage to the CNS that is observed following amphetamine and methamphetamine (METH) administration is known to be linked to increased formation of free radicals. This increase could be, in part, related to mitochondrial dysfunction and/or cause damage to the mitochondria, thereby leading to a failure of cellular energy metabolism and an increase in secondary excitotoxicity. The actual neuronal damage that occurs with METH‐induced toxicity seems to affect dopaminergic cells in particular. METH‐induced toxicity is related to an increase in the generation of both reactive oxygen (hydroxyl, superoxide, peroxide) and nitrogen (nitric oxide) species. Peroxynitrite (ONOO−), which is a reaction product of either superoxide or nitric oxide, is the most damaging radical. It can be reduced by antioxidants such as selenium, melatonin, and the selective nNOS inhibitor, 7‐nitroindazole. METH‐induced toxicity has been previously shown to increase production of the peroxynitrite stress marker, 3‐nitrotyrosine (3‐NT), in vitro, in cultured PC12 cells, and also in vivo, in the striatum of adult male mice. Pre‐ and post‐treatment of mice with l‐carnitine (LC) significantly attenuated the production of 3‐NT in the striatum after METH exposure. LC is a mitochondriotropic compound in that it carries long‐chain fatty acyl groups into mitochondria for β‐oxidation. It was shown also to play a protective role against various mitochondrial toxins, such as 3‐nitropropionic acid. The protective effects of LC against METH‐induced toxicity could be related to its prevention of possible metabolic compromise produced by METH and the resulting energy deficits. In particular, LC may be maintaining the mitochondrial permeability transition (MPT) and modulating the activation of the mitochondrial permeability transition pores (mPTP), especially the cyclosporin‐dependent mPTP. The possible neuroprotective mechanism of LC against METH‐toxicity and the role of the mitochondrial respiratory chain and the generation of free radicals and their subsequent action on the MPT and mPTP are also being examined using an in vitro model of NGF‐differentiated pheochromocytoma cells (PC12). In preliminary experiments, the pretreatment of PC12 cells with LC (5 mM), added 10 min before METH (500 μM), indicated that LC enhances METH‐induced DA depletion. The role of LC in attenuating METH‐evoked toxicity is still under investigation and promises to reveal information regarding the underlying mechanisms and role of mitochondria in the triggering of cell death.

Effects of Metabolic Modifiers Such as Carnitines, Coenzyme Q10, and PUFAs against Different Forms of Neurotoxic Insults: Metabolic Inhibitors, MPTP, and Methamphetamine

Abstract: A number of strategies using the nutritional approach are emerging for the protection of the brain from damage caused by metabolic toxins, age, or disease. Neural dysfunction and metabolic imbalances underlie many diseases, and the inclusion of metabolic modifiers may provide an alternative and early intervention approach that may prevent further damage. Various models have been developed to study the impact of metabolism on brain function. These have also proven useful in expanding our understanding of neurodegeneration processes. For example, the metabolic compromise induced by inhibitors such as 3‐nitropropionic acid (3‐NPA), rotenone, and 1‐methyl‐4‐phenylpyridinium (MPP+) can cause neurodegeneration in animal models and these models are thought to simulate the processes that may lead to diseases such as Huntingtons and Parkinsons diseases. These inhibitors of metabolism are thought to selectively kill neurons by inhibiting various mitochondrial enzymes. However, the eventual cell death is attributed to oxidative stress damage of selectively vulnerable cells, especially highly differentiated neurons. Various studies indicate that the neurotoxicity resulting from these types of metabolic compromise is related to mitochondrial dysfunction and may be ameliorated by metabolic modifiers such as l‐carnitine (L‐C), creatine, and coenzyme Q10, as well as by antioxidants such as lipoic acid, vitamin E, and resveratrol. Mitochondrial function and cellular metabolism are also affected by the dietary intake of essential polyunsaturated fatty acids (PUFAs), which may regulate membrane composition and influence cellular processes, especially the inflammatory pathways. Cellular metabolic function may also be ameliorated by caloric restriction diets. L‐C is a naturally occurring quaternary ammonium compound that is a vital cofactor for the mitochondrial entry and oxidation of fatty acids. Any factors affecting L‐C levels may also affect ATP levels. This endogenous compound, L‐C, together with its acetyl ester, acetyl‐l‐carnitine (ALC), also participates in the control of the mitochondrial acyl‐CoA/CoA ratio, peroxisomal oxidation of fatty acids, and production of ketone bodies. A deficiency of carnitine is known to have major deleterious effects on the CNS. We have examined L‐C and its acetylated derivative, ALC, as potential neuroprotective compounds using various known metabolic inhibitors, as well as against drugs of abuse such as methamphetamine.

The Action of Acetyl‐l‐Carnitine on the Neurotoxicity Evoked by Amyloid Fragments and Peroxide on Primary Rat Cortical Neurones

Abstract: The amyloid β‐peptides have been implicated in the excitotoxic mechanism of neuronal injury in the pathogenesis of Alzheimers disease. In this paper we examine the effect of different amyloid fragments (β A1–40, A1–28, and A25–35), as well as potential neuroprotective compounds on rat cortical neuron viability. Exposure of neurones to β A25–35 or A1–40 at concentrations as low as 1 μg/ml inhibited, significantly, the MTT response and this level of inhibition was similar after 24‐h or three‐day exposure. Furthermore, the level of inhibition was not affected by the presence or absence of 5% horse serum in the medium. Preexposure (10 min) of neurones to ALC at concentrations of 0.1, 1, 5, and 10 mM attenuated the inhibition of the MTT response caused by β A25–35 (50 μg/ml) in serum free medium for 24 h. The treatment of cells with vitamin E (100 μM), catalase (4 mg/ml), NGF (0.1 and 10 ng/ml), or cycloheximide (0.1 μg/ml) significantly restored the MTT response that was inhibited by β A25–35. The mechanism for the protective actions of these compounds against β A25–35 toxicity is not clear but may involve free radical scavenger action and preservation of energy production, although other mechanisms, especially for ALC, such as a direct effect on A‐β interaction with charged anionic phospholipids and/or stabilizing action on membranes, are also possible.

Role of Mitochondrial Dysfunction in Neurotoxicity of MPP:+: Partial Protection of PC12 Cells by Acetyl‐l‐Carnitine

Abstract: The damage to the central nervous system that is observed after administration of either methamphetamine (METH) or 1‐methyl‐4‐phenylpyridinium (MPP+), the neurotoxic metabolite of 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), is known to be linked to dopamine (DA). The underlying neurotoxicity mechanism for both METH and MPP+ seem to involve free radical formation and impaired mitochondrial function. The MPP+ is thought to selectively kill nigrostriatal dopaminergic neurons by inhibiting mitochondrial complex I, with cell death being attributed to oxidative stress damage to these vulnerable DA neurons. In the present study, MPP+ was shown to significantly inhibit the response to MTT by cultured PC12 cells. This inhibitory action of MPP+ could be partially reversed by the co‐incubation of the cells with the acetylated form of carnitine, acetyl‐l‐carnitine (ALC). Since at least part of the toxic action of MPP+ is related to mitochondrial inhibition, the partial reversal of the inhibition of MTT response by ALC could involve a partial restoration of mitochondrial function. The role carnitine derivatives, such as ALC, play in attenuating MPP+ and METH‐evoked toxicity is still under investigation to elucidate the contribution of mitochondrial dysfunction in mechanisms of neurotoxicity.

Links between nutrition, drug abuse, and the metabolic syndrome.

Abstract:  Nutritional deficiency in combination with drug abuse may increase risk of developing the metabolic syndrome by augmenting cell damage, excitotoxicity, reducing energy production, and lowering the antioxidant potential of the cells. We have reviewed here the following points: effects of drugs of abuse on nutrition and brain metabolism; effects of nutrition on actions of the drugs of abuse; drug abuse and probability of developing metabolic syndrome; role of genetic vulnerability in nutrition/drug abuse and brain damage; and the role of neuroprotective supplements in drug abuse. Nutrition education is an essential component of substance abuse treatment programs and can enhance substance abuse treatment outcomes. The strategies available, in particular the nutritional approach to protect the drug abusers from the metabolic syndrome and other diseases are discussed.

Metabolic syndrome in drug abuse.

Drug abuse is associated with significant health risk. Whether drug abusers are at a higher risk of suffering the metabolic syndrome is not widely known. The metabolic syndrome is a cluster of metabolic abnormalities, including hyperinsulinemia, hypertension, dyslipidemia, and abdominal obesity, and is probably triggered by initial imbalances at the cellular level in various critical metabolic pathways. These initially small metabolic imbalances are believed to cascade with time and lead to larger problems. Some indications that drug abuse may increase the risk of the metabolic syndrome include the following: